1. Field of the Invention
The present invention relates to scan conversion systems and, more particularly, to scan conversion systems for use in ultrasound imaging.
2. Description of the Related Art
In ultrasound imaging systems, ultrasound signals are transmitted into a patient's body via a transducer array. Reflected ultrasound signals are received at transducer elements which convert the reflected ultrasound signals back into electronic signals. In a general ultrasound system, the ultrasound energy or received echo intensity data is received as a sector scan of a patient area. The received beam pattern is a group of polar coordinate vectors. Since display devices such as CRTs (cathode ray tubes) are raster (x-y) coordinate devices, the received data must be converted into raster format, i.e., the received data must be converted from a polar coordinate system to an (x-y) coordinate system for display.
In typical systems, raster points on the display are assigned x-y coordinate values. For each raster point, an inverse transformation is performed to identify the polar coordinate input samples closest to the particular raster points. Once the closest polar coordinate points are found, the values associated at that polar coordinate point are assigned to the appropriate raster domain point. Since there are typically more raster points than polar coordinate points, the output raster point does not necessarily exactly coincide with a given input polar coordinate point. Thus, an interpolation must occur between polar coordinate points in order to assign an appropriate point to a raster point of interest. This process must be performed for every point in the raster display.
As is well known, polar and rectangular coordinates conversions may be accomplished using the following relation: ##EQU1## Thus, scan converters (the portions of ultrasound imaging devices which transform the echo intensity from the vector domain to the raster domain) typically require an inverse tangent function to trace back the location of the display pixel in its original azimuth direction and also a square root function to determine the distance R along the beam direction. Whenever the location of the pixel is determined, its value will be calculated from interpolation by using its neighborhood sample points in the polar domain. The arctangent and the square root functions introduce a bottleneck in system performance.
The scan conversion in prior ultrasound devices has been accomplished either through the straightforward coordinates transformation set forth above or through storing precomputed relationships between the raster pixels and a set of vector sample locations while using large memories. Each such method, however, still relies on the computationally intensive arctangent and square root functions.
Accordingly, there is a need for an ultrasound scan conversion method which overcomes these drawbacks of the prior art. There is a still further need for an ultrasound scan conversion method whereby the computationally intensive arctangent and square root functions may be avoided. There is a yet further need for a faster ultrasound scan conversion method.